Met. Mater. Int., Vol. 21, No. 3 (2015), pp. 561~568 doi: 10.1007/s12540-015-4376-z
Correlation Between Reflectance and Photoluminescent Properties of Al-rich ZnO Nano-Structures 1
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Firoz Khan , Seong-Ho Baek , Nafis Ahmad , Gun Hee Lee , Tae Hoon Seo , Eun-kyung Suh3, and Jae Hyun Kim1, * 1
Energy Research Division, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1 Sang-Ri, Hyeonpung-Myeon, Dalseong-gun, Daegu 711-873, Republic of Korea 2 Applied Science and Humanities, Galgotias College of Engineering and Technology, Knowledge Park, Phase II Greater Noida, U.P. -201-306, India 3 School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea (received date: 10 August 2014 / accepted date: 14 October 2014) Al rich zinc oxide nano-structured films were synthesized using spin coating sol-gel technique. The films were annealed in oxygen ambient in the temperature range of 200-700 °C. The structural, optical, and photoluminescence (PL) properties of the films were studied at various annealing temperatures using X-ray diffraction spectroscopy, field emission scanning electron microscopy, photoluminescence emission spectra measurement, and Raman and UV-Vis spectroscopy. The optical band gap was found to decrease with the increase of the annealing temperature following the Gauss Amp function due to the confinement of the exciton. The PL peak intensity in the near band region (INBE) was found to increase with the increase of the annealing temperature up to 600 °C, then to decrease fast to a lower value for the annealing temperature of 700 °C due to crystalline quality. The Raman peak of E2 (low) was red shifted from 118 cm-1 to 126 cm-1 with the increase of the annealing temperature. The intensity of the second order phonon (TA+LO) at 674 cm-1 was found to decrease with the increase of the annealing temperature. The normalized values of the reflectance and the PL intensity in the NBE region were highest for the annealing temperature of 600 °C. A special correlation was found between the reflectance at λ = 1000 nm and the normalized PL intensity in the green region due to scattering due to presence of grains. Keywords: Nanostructured materials, sol-gel, optical properties, X-ray diffraction, photoluminescence
1. INTRODUCTION In recent years, undoped zinc oxide (ZnO) has received much attention because of its potential uses in many applications such as antireflection coating in solar cells, UV light emitting diodes, laser diodes, varistors, and transparent conducting films [1-6]. ZnO films show two emission peaks in the UV and visible regions. The UV emission peak is due to a bound exciton, while the visible (blue-green) emission may be due to different intrinsic defects such as zinc vacancies, interstitial oxygen (Oi-), and oxide anti-site defects (OZn) forming local deep levels in the band gap of ZnO [7,8]. Doped ZnO films show stable electrical and optical properties. Among the doped ZnO films, Al-doped ZnO films have good electrical conductivity and optical transmittance in the visible and near-infrared regions [9,10]. Several techniques are used for *Corresponding author:
[email protected] KIM and Springer
the deposition of Al-doped ZnO films [11-17]. Among those techniques, sol-gel has many advantages: it is easy, inexpensive, and suitable for mass fabrication. The structural, optical, and electrical properties of Al-doped ZnO films strongly depend on the doping concentration, annealing temperature, and annealing ambient. There are several reports available in the literature that consider the Al doping [18], annealing temperature [19], and annealing ambient [20,21] dependence of some structural, optical, and electrical properties of Al-doped ZnO films. Lee et al. [18] have studied the effect of Al doping concentration on the structural and electrical properties of atomic layer deposited (ALD) Al-doped ZnO films. Tun et al. [19] have studied the effect of annealing temperature on some of the structural and electrical properties of Al rich zinc oxide (AZO) films coated on gallium nitride by dc sputtering technique. They [19] have found that the crystalline quality and electrical mobility continuously increase with increases of the annealing temperature up to 800 °C. Previously, it has been determined that the air annealed Al rich ZnO (AZO)
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with an Al/Zn molar ratio > 20% can be used as an antireflection coating [22] on the front surface of silicon solar cells. However, hydrogen annealed AZO films passivate the p-type silicon surface [23]. Later on, such films have been used as passivating layers of p-back surfaces of the bifacial [24], and passivated emitter and rear cell [PERC] solar cells [25]. Recently, we [26] investigated the influence of the Al/Zn ratio on the passivation properties of AZO films. Therefore, it is also important to study these films’ structural, optical, and photoluminescence (PL) properties at various annealing temperatures. In this paper, we study the effects of annealing temperature on the structural, optical, and PL properties and their correlations of nano-structured AZO films deposited directly onto quartz and silicon substrates using the sol-gel technique. The structural, optical, and PL properties of the films are studied at various annealing temperatures using X-ray diffraction spectroscopy, field emission scanning electron microscopy, photoluminescence emission spectra measurement, and Raman and UV-Vis spectroscopy. Some correlations have been investigated between reflectance and PL emission spectra. A relation between the optical band-gap (Eg) and the annealing temperature has been observed.
2. EXPERIMENTAL PROCEDURES 2.1. Synthesis of AZO films AZO films were synthesized using solutions that were prepared by dissolving zinc acetate dihydrate and aluminium nitrate nonahydrate in a 20% molar ratio in ethanol. A small amount of mono ethanolamine (MEA) was added drop wise as a stabilizer to clear the solution. After proper mixing using a magnetic stirrer for 8 hrs at 60 °C the solutions were kept for one day at room temperature. Glass and chemically mechanically polished silicon substrates were first cleaned in a Piranha (H2SO4:H2O2) solution, and then the films were coated on thoroughly cleaned substrates by spin-coating of the AZO solution. The spin-coated AZO films were initially dried in air at 120 °C for 30 min; then, the samples were annealed in a conventional furnace at different temperatures in the temperature range of 200-700 °C in oxygen ambient for 30 min. The AZO films were annealed at 200, 400, 500, 600, and 700 °C. Henceforth, we shall refer to the AZO films as AZO-200, AZO-400, AZO-500, AZO-600, and AZO-700 for annealing temperatures 200, 400, 500, 600, and 700 °C, respectively, in the text. 2.2. Characterization of AZO films X-ray diffraction (XRD) patterns of the thin films were recorded in the 2θ range from 20° to 80° using Cu/Kα radiation (λ = 1.5406Å) using an X-ray diffractometer, the Empyrean Pananalytical HR-XRD system. Field Emission Scanning Electron Microscopy (FE-SEM) micrographs of AZO films coated on silicon substrates were taken under 150KX mag-
nification using a Hitachi FE-SEM, Model S-4800. Photoluminescence measurements were performed using a Jasco spectrofluorometer Model FP-6500 with a Xenon flash lamp and gratings to provide the required excitation. The emission spectra were recorded in fluorescence mode over a range of 360-750 nm at room temperature at an excitation wavelength of 240 nm. Reflectance and transmittance of all the AZO films were measured in the wavelength range of 300-1200 nm using a PerkinElmer UV-Vis-NIR spectrometer Model Lamda 750.
3. RESULTS AND DISCUSSION 3.1. Structural properties The XRD results show the presence of peaks due to the reflections of the planes from a wurtzite type of ZnO structure. Figure 1 shows the XRD patterns of the AZO films annealed at various temperatures in the range of 200-700 °C under oxygen ambient. The XRD pattern of AZO-200 films confirms that these films are amorphous in nature. The XRD results show that the films changed from an amorphous to a polycrystalline structure at annealing temperatures more than 400 °C [27]. Nishio et al. [28] observed from TG-DTA measurements that there was a large exothermic peak with a large weight loss at ~390 °C. Among the crystalline films, the AZO-400 and AZO-500 films exhibited a lesser degree of crystallinity. The crystallinity is known to increase with annealing temperature up to 600 °C; it then starts to decrease for higher values of annealing temperature. Similar observations have been made by Nishio et al. [28]. In case of AZO-500 film, the oxygen is chemisorbed in the pores on the AZO surface. By increasing the annealing temperature, desorption of oxygen takes place and the crystallinity increases up to annealing temperature of 600 °C [27]. The crystalline phase present in these films
Fig. 1. XRD patterns of AZO films annealed at 200, 400, 500, 600, and 700 °C in oxygen ambient along with reference XRD patterns (black line corresponding to ZnO and red line corresponding to Al2O3).
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is identified as the wurtzite hexagonal crystal structure of ZnO, as per JCPDS 36-1451 [29]. In all the crystalline films, i.e., AZO-400, AZO-500, AZO-600, and AZO-700 films, Bragg peaks corresponding to hexagonal ZnO appeared at 2θ = 31.82, 34.46, 35.92, 47.6, 56.63, 62.88, and 68.05° from the (100), (002), (101), (102), (110), (103), and (112) reflections, respectively. Three additional peaks have been observed in the AZO-700 sample at 2θ = 44.84, 59.35, and 65.12° from Bragg’s reflection planes (003), (241), and (312) of Al2O3, respectively. For annealing temperatures higher than 600 °C, it is possible that the bond between Zn and O breaks and the crystallinity starts to decrease. It is noteworthy that the crystalline nature of the AZO-500 film is slightly different owing to the very low intensity of the peaks at 2θ = 31.82, 34.46, and 35.92°, which are more intense peaks in all the crystalline AZO (AZO-400, AZO-500, AZO-600, and AZO-700) samples. The very low intensity peak of AZO-500 at 2θ = 34.46° along the c-axis orientation is indicative of the lowest conductivity among all the samples. This may be due to the doping level of the Al being very high (~20 at%); therefore, the refraction peak at 2θ = 34.46° for ZnO is very weak in the X-ray diffraction pattern of the AZO-500 film [30]. An energy dispersive X-ray (EDX) elemental analysis was conducted for determination of the atomic percentages of Al, Zn, and O. The variations of the Al/O, ZnO, and Al/ Zn atomic ratios are shown in Fig. 2. It can be seen that the
Fig. 2. Al/O, Zn/O, and Al/Zn atomic ratios calculated from EDX analysis of the AZO films annealed at various temperatures.
Al/Zn ratio is the same for all the samples. Among all the crystalline samples, the Al/O and Zn/O atomic ratios are found to be lowest for AZO-500. A maximum value of the Zn/O atomic ratio is obtained for AZO-600. For the annealing temperature of 700 °C, the Zn/O atomic ratio is found to decrease, whereas the Al/O ratio remains the same as that for AZO-600. This confirms that the crystalline quality of AZO decreases with the decrease of the Zn/O atomic ratio and, hence, that the AZO-500 film has the lowest crystalline quality. In all likelihood, the combination of the Al/Zn atomic ratio of 20 % and the annealing temperature of 500 °C are responsible for the lowest peak’s intensity. Importantly, a preferred orientation is seen along the (101) plane in all the crystalline AZO samples, whereas Verma et al. [22] have observed a preferred orientation along the (100) plane of air annealed AZO film. Valle et al. [31] have observed that there is a preferred AZO crystal growth direction along the (100) plane for Al/Zn atomic ratios up to 3%. However, three principal peaks along the (100), (002), and (101) planes are observed for higher Al doping concentrations. Ohyama et al. [32] have obtained highly crystal growth oriented along the (002) reflection plane for the 0.5% Al/Zn atomic ratio of AZO films. In order to determine the effect of annealing on the lattice parameters, a and c are calculated using the formula a = -----------------3Sin and c = ----------Sin [33]. The crystallite size has been estimated using Scherrer’s formula [34]. The biaxial stress of the films can be calculated using the relation σ = -453.6×109 [(c-c0)/c0], where c0 = 0.5206 nm in the strain-free lattice parameter [35]. The calculated values of a and c, and the grain sizes (D) of all the films, are listed in Table 1. From Table 1, it can be seen that the 2θ value decreases from 36.35° for the annealing temperature of 400 °C to a minimal value of 36.07° for the annealing temperature of 500 °C. For higher annealing temperatures this value started to increase and attained a value of 36.32° for an annealing temperature of 700 °C. The spacing between the two adjacent (101) planes (d) is found to increase from 0.4375 nm (for 400 °C) to its highest value of 0.4391 nm (for 500 °C); this value decreased for higher annealing temperatures. A d value of 0.4377 nm is obtained for an annealing temperature of 700 °C. It has been found that the grain sizes of the AZO films increased (reducing the grain boundaries) with the increase of the annealing temperature. The surface morphologies of the as-deposited AZO films and the films annealed at various temperatures in the tem-
Table 1. Structure and geometry parameters of AZO films annealed at 200, 400, 500, 600, and 700 °C in oxygen ambient Annealing temperature (°C) 200 400 500 600 700
2Theta
hkl
d (nm)
Structure
-36.35 36.07 36.27 36.32
-101 101 101 101
-0.4375 0.4391 0.4380 0.4377
Hexagonal
Lattice parameter (nm) a c --0.4952 0.2859 0.2880 0.4956 0.2865 0.4962 0.2861 0.4966
Crystalline size D (nm) -9.13 9.86 14.62 12.08
Stress 10 (Pa) × 10 -2.213 2.178 2.126 2.091
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Fig. 4. Reflectance spectra of AZO films annealed at 200, 400, 500, 600, and 700 °C in oxygen ambient in the wavelength range of 300-1200 nm.
Fig. 3. FESEM micrographs of the (a) as deposited AZO films, (b) AZO-200, (c) AZO-400, (d) AZO-500, (e) AZO-600, and (f) AZO-700 in oxygen ambient.
perature range of 200-700 °C are shown in Fig. 3. Figures 3a, 3b, 3c, 3d, 3e, and 3f show the FE-SEM images of the asdeposited AZO films and the films annealed at 200, 400, 500, 600, and 700 °C, respectively. It can be seen from Figs. 3a and 3b that these films are amorphous in nature (no grain boundary). The AZO films at higher annealing temperatures (400 °C) are crystalline; the crystalline quality increases with the increase of the annealing temperature up to 600 °C. The crystalline size increases from ~9 to ~15 nm with the increase of the annealing temperature up to 600 °C, while this value slightly drops to ~12 nm for an annealing temperature of 700 °C. 3.2. Optical properties The reflectance spectra of the AZO films (coated on Si) annealed at 200, 400, 500, 600, and 700 °C coated on Si in the wavelength range of 300-1200 nm are shown in Fig. 4. The minimum and maximum reflectance values are found in the wavelength ranges of 400-600 nm and 900-1100 nm, respectively. In our case, a minimum in the reflectance spectra for all the crystalline samples was obtained at λ1000 nm (for annealing temperatures 400 °C). The minimal reflectance value obtained at any wavelength depends on the optical thickness (a multiplication of the refractive index and the thickness) of the film and is an integer multiple of that wavelength. This wavelength value (~1000 nm) can be changed with
changes in the optical thicknesses of the films. It can be seen in Fig. 4 that the reflectance value at λ = 1000 nm decreases with the increase of the annealing temperature and, from its initial value of 26.14% for an annealing temperature of 200 °C, attains a value of 0.30% for 600 °C. The reflectance value slightly increases to a value of ~1% for the annealing temperature of 700 °C. The wavelength value for the minimum reflectance of the annealed samples is blue shifted with the annealing temperature. This blue shifting is due to the decrease of the optical thickness of the AZO film with the increase of the annealing temperature [36]. The dependency of the surface roughness on annealing temperature of AZO films, measured using atomic force microscopy (AFM), is shown in Fig. 5. The surface roughness is found to decrease from ~21 nm to ~9 nm with the increase of the annealing temperature from 400 °C to 600 °C; this value started to increase with further increases in the annealing temperature. A similar trend of variation of the reflectance (maximum) value in the wave-
Fig. 5. Surface roughness of the AZO film with annealing temperature.
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Fig. 6. Transmittance spectra of AZO films annealed at 200, 400, 500, 600, and 700 °C in oxygen ambient in the wavelength range of 300-1200 nm.
Fig. 8. Variation of optical band gap of AZO films with annealing temperature.
length range of 400-600 nm with annealing temperature was also observed. This confirms that the variations of the reflectance value at the maxima and minima are due to the surface roughness and optical thicknesses of the films. The transmittance of the film is found to gradually decrease with the increase of the annealing temperature up to 700 °C, as shown in Fig. 6. All the films coated on the glass substrate have average transmittances greater than 75% in the 400-1200 nm wavelength range. A linear decrease in average transmittance is found in the temperature range of 400-700 °C, as shown in the inset of Fig. 6. This value decreases from ~84% at an annealing temperature of 400 °C to ~76% at an annealing temperature of 700 °C. These transmittance values have been used to determine the optical band gap Eg of the films from intercepts on the energy axis after extrapolation of the straight-line section in the high energy region of the (αhν)2 vs. hν curve, where hν is the photon energy and α is the absorption coefficient of the film (as shown in Fig. 7). The variation of the value of Eg of the AZO film with annealing
temperature is shown in Fig. 8. The value of Eg decreases with annealing temperature and attains a minimal value of 3.22 eV for the annealing temperature of 700 °C (this is in contrast to the value of 3.36 eV for the annealing temperature of 200 °C). The narrowing of Eg up to 700 °C is due to the increase of the crystalline size of the AZO film. The phenomenon of the exciton being bound to the nano-crystallite and depending on the size of the particle (crystallite) is known as the quantum confinement effect. The stress of the film is also varied in the same manner (as listed in Table 1). The experimental data of Eg with annealing temperature is Gauss Amp fitted using origin software to determine the relation of the value of Eg of the AZO film to annealing temperature. The determined relation is given by Eq. (1).
Fig. 7. Direct optical energy band-gap evaluation of AZO films annealed at various temperatures in oxygen ambient.
2
t – tc E t = E 0 + A.exp – --------------- 2.w
(1)
where E(t) is the optical band-gap of the AZO film at the annealing temperature t and E(0) is the optical band-gap of the as deposited film. A, w, and tc are constants. The values of E(0), A, w, and tc are 3.37 eV, -0.1512 eV, 208.96 °C, and 685.22 °C, respectively. 3.3. Photoluminescence properties Figure 9 shows the PL spectra of the AZO films coated on Si and annealed at 200, 400, 500, 600, and 700 °C for the excitation wavelength (λex) = 240 nm. The PL peak in the UV region is found in the emission wavelength range of 388395 nm. This is also called near band edge emission (NBE) [37]. The PL emission in the NBE region originates from the free exciton emission [38]. The PL intensity in this region increases with the annealing temperature up to 600 °C. A sudden decrease in PL intensity in the NBE region has been observed for an annealing temperature of 700 °C. The PL emission in the NBE
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Fig. 9. PL spectra of AZO films annealed at various temperature in the temperature range of 200-700 °C in oxygen ambient.
region is indicative of the crystalline quality. A higher crystalline quality (low number of defects) AZO film shows a higher PL intensity in the NBE region. This confirms that the crystalline quality increases with the increase of the annealing temperature up to 600 °C and then decreases for higher annealing temperatures. An similar observation has been made using XRD analysis of the films annealed at different temperatures. The variation of the PL peak intensities in the NBE region (INBE) of the AZO films is reflected by the variation of XRD peak intensities. The PL emission in the visible (blue and/or green) regions is due to defects in the film. The PL peak position in the NBE region is red shifted with the increase of the annealing temperature. This red shifting in the NBE emission peak is due to an exciton confined in the AZO nanoparticles or to an exciton bound to an impurity located at the nanoparticle surface [39]. In our case, the crystalline size is found to increase with the increase of the annealing temperature, which affects the energy level of the confined exciton. There are two peaks found in the emission wavelength ranges of 430-450 nm (blue region) and 520-550 nm (green region). The PL intensities in both the blue (IB) and green (IG) regions are found to decrease with the increase of the annealing temperature up to 600 °C. 3.4. Raman spectra study The Raman scattering spectra of the AZO films at room -1 temperature are shown in Fig. 10. Two peaks, at ~301 cm with -1 low intensity and ~520 cm with very high intensity, are observed for the silicon substrate [40]. The E2 optical mode is observed at -1 -1 118-126 cm (due to low frequency) and 405-414 cm (due to high frequency) [41]. E2 (low) is associated with the vibration of the heavy Zn sub lattice [42]. The Fourier transform infrared spectroscopy measurement results also showed an absorp-1 -1 tion peak in the range of 420 cm to 520 cm which standard peak ZnO [37,43]. The peak in the wavenumber range of -1 405-414 cm can be assigned to the high frequency branch of
Fig. 10. Raman spectra of AZO films annealed at 200, 400, 500, 600, and 700 °C in oxygen ambient.
the E2 mode [E2 (high)] of ZnO, which is the strongest mode in the wurtzite crystal structure. The strong E2 (high) mode indicates a good crystallinity. Lo et al. [42] have observed that the E2 (high) mode shifted to 438 cm-1 for Al doping for 4 atomic % from 442 cm-1 for undoped ZnO. In our case, the doping level of Al is very high (~20 atomic %). Therefore, the E2 (high) mode is observed at 405-414 cm-1. The intensity of the E2 (high) mode is found to increase with the increase of the annealing temperature; the highest intensity is observed for a temperature of 600 °C. With any further increase in the annealing temperature, the intensity of this mode drops. One usual mode of A1 (LO) at ~618 cm-1 is observed in the all the AZO samples. One second order phonon (TA+LO) mode has also been observed at ~674 cm-1 [41]. The TA+LO mode is activated due to Al-doping and the intensity of this mode is found to decrease with the increase of the annealing temperature. 3.5. Correlation between reflectance and PL The dependency of the normalized XRD peak intensity on the annealing temperature, along with the crystalline size, is shown in Fig. 11. The maximal values of the XRD peak intensities corresponding to the (100), (002), and (101) planes and the crystalline sizes of the AZO films are found for the annealing temperature of 600 °C. The variations of the normalized PL intensity ratio IG/INBE and the reflectance at the wavelength of 1000 nm with the annealing temperature are shown in Fig. 12. A sudden fall in the IG/INBE ratio has been observed with the increase of the annealing temperature; from its initial value of 0.98 for the annealing temperature of 200 °C, this ratio attains a minimal value of 0.08 for the annealing temperature of 600 °C. This observation also indicates that the AZO-600 film has the lowest number of defects (highest degree of crystallinity) [42]. The IG/INBE ratio again starts to increase for higher annealing temperatures and attains a value of 0.27 at the annealing tem-
Correlation Between Reflectance and Photo-luminescent Properties
Fig. 11. Variation of normalized XRD peak intensity and crystalline size with annealing temperature.
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pared by sol-gel process have been studied for the temperature range of 200-700 °C. The crystalline quality was found to improve with the increase of the annealing temperature up to 600 °C; this crystalline quality then started to degrade for higher annealing temperatures. XRD and FE-SEM results confirmed that the crystalline quality and the grain size increase with the increase of the annealing temperature. A minimal 2θ value was observed for a Bragg’s diffraction peak corresponding to the (101) plane for an annealing temperature of 500 °C. The average transmittance values were found to decrease with the increase of the annealing temperature up to 700 °C. The PL intensity in the NBE region decreased with the increase of the annealing temperature up to 600 °C. The PL positions in the NBE and green regions were red shifted with annealing temperature due to the increase of the crystalline size and the improvement in the crystalline quality, respectively. The optical band-gap was found to decrease with the increase of the annealing temperature, following the Gauss Amp function. The normalized maximal value of the reflectance, and the PL intensity, were obtained for the annealing temperature of 600 °C. However, the peak positions of reflectance and PL in the near band region (NBE) were found to increase with the increase of the annealing temperature. A special correlation between the IG/INBE ratio and the reflectance value at λ = 1000 nm and the annealing temperature was found. This correlation may be due to the variation of the grains (grain boundaries) of the nano-structured AZO films with annealing temperature.
ACKNOWLEDGEMENTS Fig. 12. Variation of reflectance value at λ = 1000 nm and normalized intensity ratio of green and near band region with annealing temperature.
perature of 700 °C. Similarly, the reflectance value of the AZO films at λ = 1000 nm decreases with annealing temperature and, from its value 24.7% at 400 °C, attains a minimal value of 0.3% at 600 °C. For higher annealing temperatures, the reflectance again increases and finally attains a value of ~1% at 700 °C. Thus in this study, some correlation between the variations of the PL and the reflectance and the annealing temperature is found. The increase of the crystalline size is indicative of a lowering in the number of grain boundaries and hence a lowering of the number of surface defects, which reduces the PL intensity in the green region. The scattering due to grains is reduced with the increase of the annealing temperature and a minimal value of reflectance at λ = 1000 nm is obtained at 600 °C.
4. CONCLUSIONS The effects of annealing temperature on the structural, optical, and photoluminescent properties of AZO films pre-
This work was supported by the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0001649), and Ministry of Science, ICT & Future Planning (MSIP). Also partially funded by the Ministry of Trade, Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT), Dae Gyeong Institute for Regional Program Evaluation (DGIRPE) through the Leading Industry Development for Economic Region and partially funded by the Energy International Collaboration Research & Development Program of the Ministry of Knowledge Economy (MKE) (2011-8520010050).
REFERENCES 1. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, Science 292, 1897 (2001). 2. J. Zhou, Y. Wang, F. Zhao, Y. Wang, Y. Zhang, and L. Yang, J. Lumin. 119-120, 248 (2006). 3. H. Rensmo, K. Keis, H. Lindström, S. Södergren, A. Solbrand, A. Hagfeldt, S.E. Lindquist, L.N. Wang, and M. Muhammed, J. Phys. Chem. B 101, 2598 (1997).
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Firoz Khan et al.
4. W. Z. Wang, B. Q. Zeng, J. Yang, B. Poudel, J. Y. Huang, M. J. Naughton, and Z. F. Ren, Adv. Mater. 18, 3275 (2006). 5. H. Kind, H. Yan, B. Messer, M. Law, and P. Yang, Adv. Mater. 14, 158 (2002). 6. E. S. Jang, J. H. Won, S. J. Hwang, and J. H. Choy, Adv. Mater. 18, 3309 (2006). 7. B. Lin, Z. Fu, and Y. Jia, Appl. Phys. Lett. 79, 943(1-3) (2001). 8. A. E. Jimenez-Gonzalez, J. A. S. Urueta, and R. Suarez-Parra, J. Cryst. Growth 192, 430 (1998). 9. H. Kim, A. Pique, J.S. Horwitz, H. Murata, Z. H. Kafafi, C. M. Gilmore, and D. B. Chresey, Thin Solid Films 377-378, 798 (2000). 10. J. H. Lee and B. O. Park, Mater. Sci. Eng. B 106, 242 (2004). 11. B. M. Ataev, A. M. Bagamadova, V. V. Mamedov, A. K. Omaev, and M. R. Rabadanov, J. Cryst. Growth 198-199, 1222 (1999). 12. D. Song, P. W. Denborg, W. Chin, and A. G. Aberle, Sol. Ener. Mater. Sol. Cells 73, 1 (2002). 13. J. H. Lee, K.H. Ko, and B. O. Park, J. Cryst. Growth 247, 119 (2003). 14. Y. Liu, Q. Li, and H. Shao, J. Alloy. Compd. 485, 529 (2009). 15. J. Cembrero, A. Elmanouni, B. Hartiti, M. Mollar, and B. Marí, Thin Solid Films 451-452, 198 (2004). 16. K. Bahedi, M. Addou, M. E. Jouad, Z. Sofiani, S. Bayoud, M. E. L. Haouti, and J. Ebothe, Proceeding of the ICTON Mediterranean Winter Conference, 3rd (eds. Marek Jaworski, Marian Marciniak), pp.1-4, National Inst. Telecomm. Warsaw, Poland (2009). 17. P. Banerjee, W. J. Lee, K. R. Bae, S. B. Lee, and G. W. Rubloff, J. Appl. Phys. 108, 043504 (1-7) (2010). 18. D. J. Lee, H. M. Kim, J. Y. Kwon, H. Choi, S. H. Kim, and K. B. Kim, Adv. Func. Mat. 2, 448 (2011). 19. C. J. Tun, J. K. Sheu, M. L. Lee, C. C. Hu, C. K. Hsieh, and G. C. Chi, J. Electrochem. Soc. 153, G296 (2006). 20. L. B. Duan, X. R. Zhao, J. M. Liu, W. C. Geng, H. Y. Xie, and H. N. Sun, Phys. Scr. 85, 0357091 (2012). 21. H. Chen, Y. H. Jeong, and C. B. Park, Trans. Electric. Electron. Mater. 10, 58 (2009). 22. A. Verma, F. Khan, D. Kumar, M. Kar, B. C. Chakravarty, S. N. Singh, and M. Husain, Thin Solid Films 518, 2649 (2010). 23. F. Khan, Vandana, S. N. Singh, M. Husain, and P. K. Singh, Sol. Ener. Mater. Sol. Cells 100, 57 (2012).
24. F. Khan, S. H. Baek, S. N. Singh, P. K. Singh, and J. H. Kim, Sol. Ener. 97, 474 (2013). 25. F. Khan, S. H. Baek, A. Mobin, and J. H. Kim, Sol. Ener. 101, 265 (2014). 26. F. Khan, S. H. Baek, S. N. Singh, P. K. Singh, M. Husain, and J. H. Kim, Solar Ener. 110, 595 (2014). 27. W. Tang and D. C. Cameron, Thin Solid Films 238, 83 (1994). 28. K. Nishio, S. Miyake, T. Sei, Y. Watanabe, and T. Tsuchiya, J. Material. Science 31, 3651 (1996). 29. J. D. Hanawalt, H. W. Rinn, and L. K. Frevel, Anal. Chem. 10, 475 (1938). 30. R. Wahab, Y. S. Kim, and H. S. Shin, Met. Mater. Int. 16, 767 (2010). 31. G. G. Valle, P. Hammer, S. H. Pulcinelli, and C. V. Santilli, J. Eur. Ceram. Soc. 24, 1009 (2004). 32. M. Ohyama, H. Kozuka, and T. Yoko, J. Am. Ceram. Soc. 81, 1622 (1998). 33. C. Suryanarayana and M. G. Norton, X-ray Diffraction: A Practical Approach, pp.97-152, Plenum Press, New York (1998). 34. B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction, 3rd edition, p.295, Prentice Hall, Upper Saddle River, NJ (2001). 35. M. K. Puchert, P. Y. Timbrell, and R. N. Lamb, J. Vac. Sci. Technol. A 14, 2200 (1996). 36. D. Verma, F. Khan, S. N. Singh, and P. K. Singh, Sol. Ener. Mater. Sol. Cells 95, 30 (2011). 37. F. Khan, S. Ameen, M. Song, and H. S. Shin, J. Lumin. 134, 160 (2013). 38. F. Khan, S. Ameen, M. Song, M. Husain, A. Mobin, and H. S. Shin, Met. Mater. Int. 19, 245 (2013). 39. V. A. Fonoberov and A. A. Balandin, Appl. Phys. Lett. 85, 5971 (1-3) (2004). 40. Y. Kanzawa, S. Hayashi, and K. Yamamoto, J. Phys Condens. Matter 8, 4823 (1996). 41. M. S. Jang, M. K. Ryu, M. H. Yoon, S. H. Lee, H. K. Kim, A. Onodera, and S. Kojima, Current Appl. Phys. 9, 651 (2009). 42. S. S. Lo, D. Huang, C. H. Tu, C. H. Hou, and C. C. Chen, J. Phys. D: Appl. Phys. 42, 095420 (2009). 43. V. V. Ursaki, E. Rusu, V. Zalamai, L. Sirbu, E. Monaico, and I. M. Tiginyanu, Inf. Technol. 5822, 148 (2005).
